Flame-Synthesis of Monolayer and Nano-Defective Graphene

ABSTRACT

Methods for the production of carbon-based and other nanostructures are provided.

This application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Patent Application No. 62/608,739, filed on Dec. 21, 2017.The foregoing application is incorporated by reference herein.

This invention was made with government support under Grant No.W911NF-17-1-0111 awarded by the Army Research Office and under Grant No.CBET-1249259 awarded by the National Science Foundation. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

This invention describes methods for the synthesis of carbon-basednanostructures and articles derived therefrom.

BACKGROUND OF THE INVENTION

Flame synthesis has been used for manufacturing fine powders since the1940s, when fumed silica was first mass-produced and marketed(Pratsinis, S. E. (1998) Progress Ener. Combustion Sci., 24(3):197-219).Today, flame synthesis is widely used in commercial production ofnanoparticles such as carbon blacks, pigmentary titania, zinc oxides,fumed silica, and optical fibers (Teoh, W. Y. (2013) Materials6(8):3194-3212). A typical process of synthesizing ceramic powders is byhydrolysis of chloride-based precursor vapor injected into a flame.Flame processes readily provide the high temperatures needed forgas-phase synthesis. The key reasons flame synthesis is favored bylarge-scale manufactures are its scalability and relatively low cost.

Flame synthesis is currently used in producing various advancednanomaterials, e.g., carbon nanotubes (CNTs) and nanoparticles withcomplex compositions (Li, et al. (2016) Progress Ener. Combustion Sci.,55:1-59). However, most studies still focus on the flame aerosolapproach. Although chemical vapor deposition (CVD)-type flame synthesisof nanostructured carbon and metal oxides has been demonstrated inrecent years, its ability for large production needs more development.Nevertheless, flame synthesis has the potential of extending its useinto CVD-type processes for the controllable growth of large-areananomaterials on substrates and surfaces with advantages in scalabilityand cost.

SUMMARY OF THE INVENTION

In accordance with the instant invention, methods for synthesizingcarbon-based, particularly graphene, materials or structures. In aparticular embodiment, the method comprises reacting an oxidizer (e.g.,air or O₂) and a fuel (e.g., hydrogen and/or hydrogen precursors) in oneor more non-premixed, multiple, inverse-diffusion flame burner(s). Thenon-premixed, multiple, inverse-diffusion flame burner may comprise anarray of stabilized flames that form a uniform flat-flame front withrespect to distances downstream. In a particular embodiment, themultiple, inverse-diffusion flame burner is modified wherein the burnercomprises delivery tubes for the fuel/precursor extending beyond themain burner surface. In a particular embodiment, the pyrolysis speciesexiting the multiple, inverse-diffusion flame burner are directed ontosubstrates (e.g., metal substrates such as copper) to form coatings,preforms, flakes, films, sheets, plates, discs, and the like. In aparticular embodiment, graphene structure is monolayer graphene or anano-defective (e.g., at the atomic scale) graphene. The graphenestructures may be doped (e.g., with B or N). In a particular embodiment,the graphene structures are synthesized by a method comprising a)reducing a metal substrate in the modified, multiple inverse-diffusionflame burner wherein hydrogen (H₂) is the only fuel; b) adding ahydrocarbon precursor through the fuel lines extending beyond the mainburner surface of the modified, multiple inverse-diffusion flame burnerto synthesize the graphene on the substrate; and, optionally, c)annealing the flame-synthesized graphene of step b) by running themodified, multiple inverse-diffusion flame burner wherein hydrogen (H₂)is the only fuel.

In a particular embodiment, methods of synthesizing monolayer grapheneare provided comprising a) reducing a metal substrate in a modified,multiple inverse-diffusion flame burner wherein hydrogen (H₂) is theonly fuel; b) adding a hydrocarbon precursor through the fuel linesextending beyond the main burner surface of the modified, multipleinverse-diffusion flame burner to synthesize the graphene on asubstrate; and c) annealing the flame-synthesized graphene of step b) byrunning the modified, multiple inverse-diffusion flame burner whereinhydrogen (H₂) is the only fuel.

In a particular embodiment, methods of synthesizing nano-defective(e.g., at the atomic scale) graphene are provided comprising a) reducinga metal substrate in a modified, multiple inverse-diffusion flame burnerwherein hydrogen (H₂) is the only fuel; b) adding a hydrocarbonprecursor through the fuel lines extending beyond the main burnersurface of the modified, multiple inverse-diffusion flame burner tosynthesize the graphene on the substrate; and, optionally, c) annealingthe flame-synthesized graphene of step b) by running the modified,multiple inverse-diffusion flame burner wherein hydrogen (H₂) is theonly fuel, wherein the flame-synthesized graphene of step b) has anI_(D)/I_(G) ratio of about 0.6 or higher prior to step c). In aparticular embodiment, the metal substrate is about 99.9% pure.

BRIEF DESCRIPTIONS OF THE DRAWING

FIG. 1A provides a schematic diagram of a modified multi-elementinverse-diffusion flames (m-I_(D)Fs) burner setup modified withuniform-distributed precursor tubes staged above (e.g., extendingbeyond) the main burner surface at a fixed height.

FIG. 1B provides a rasterizing 3-burner assembly across a surface tocoat/grow graphene. The first burner reduces the oxide layer; the secondburner synthesizes graphene on the surface; and the third burner annealsthe graphene to change its characteristics. A monitoring system of Ramanand Photoluminescence may follow the burners to assess the coatingquality as it is laid down/grown.

FIG. 2 provides a schematic of two-step flame synthesis of monolayergraphene using modified m-I_(D)F setup.

FIG. 3 provides typical Raman spectra of graphene sample before andafter post-growth hydrogen annealing treatment (top). A singleLorentzian fitting of a Raman spectra from monolayer graphene sample isalso provided (bottom). Substrate background signals are subtracted. Thespectra are normalized with the G band.

FIGS. 4A and 4B provide TEM images of monolayer graphene with differentresolutions. The top right inset of FIG. 4A shows the SAED pattern, andthe top left inset of FIG. 4A shows a magnified image of the hexagonalatomic lattice in the circled edge. FIG. 4C provides an AFM image ofmonolayer graphene with highlighted spots of residuals from transferprocess.

FIG. 5 provides a typical Raman spectra of few-layer graphene grown onnickel at 1000° C. (left) and a Raman spectra of few-layer graphene onSi/SiO₂ using pulsed laser deposition (PLD) at 900° C. in vacuum(right). Substrate background signals are subtracted. The spectra arenormalized with the G band.

FIG. 6 provides a Raman spectra of defective graphene with differenthydrogen annealing time. Substrate background signal is subtracted. Thespectra are normalized with the G band.

FIG. 7A provides a high-resolution TEM image of flame-synthesizedhighly-defective graphene captured by Room TemperatureScanning/Transmission Electron Microscope (FEI Talos F200X S/TEM, 200kV). FIG. 7B provides an enlarged image of the selected area (redcircled) of FIG. 7A. FIG. 7C provides an enlarged TEM image of FIG. 4A.

FIG. 8 shows band energies of flame-synthesized defective graphene withCu substrate at different post-growth hydrogen annealing time. Thebackground from Cu plasmonic resonance (580 nm) is subtracted for allthe samples.

FIG. 9 provides a graph of normalized Kubelka Munk of graphene samples;increasing time and increasing defects leads to a red-shift in the π-π*transition of the aromatic C═C bond in graphene.

FIG. 10 provides Raman spectra of ultra-smooth Cu sample before andafter hydrogen annealing. Substrate background signal is subtracted. Thespectra are normalized with the G band.

FIG. 11 provides Raman spectra of graphene grown on Cu at differenttemperatures. Substrate background signals are all subtracted. Thespectra are normalized with the G band.

FIG. 12 provides Raman spectra of graphene grown on Cu at differentJ_(CH4):J_(H2) ratios. Substrate background signals are all subtracted.The spectra are normalized with the G band.

FIG. 13 provides Raman spectra of graphene grown on Cu at differentgrowth time. Substrate background signals are all subtracted. Thespectra are normalized with the G band.

FIG. 14 provides Raman spectra of graphene grown on differentsubstrates. Substrate background signals are all subtracted. The spectraare normalized with the G band.

FIG. 15 provides Raman spectra of graphene grown on Cu using C₂H₄ andC₂H₂. Substrate background signals are all subtracted. The spectra arenormalized with the G band.

FIG. 16 provides a schematic of the substrate placed perpendicular,tilted and parallel against the flow (left) and Raman spectra ofgraphene grown on Cu with different orientations. Substrate backgroundsignals are all subtracted. The spectra are normalized with the G band.

DETAILED DESCRIPTION OF THE INVENTION

A modified multi-element inverse-diffusion flames (m-I_(D)Fs)(non-premixed) burner setup is utilized to synthesize mono-layer and/ordefective graphene on metal substrates. The defect level usingunconfined flame synthesis can be tuned by adjusting various parameterssuch as temperature, growth time, carbon precursor, and hydrogen flowrate. An effective etching phenomenon on graphene layers reduces thenumber of layers by applying a post-growth hydrogen annealing processusing the same setup, where the hydrocarbon precursor flow is turnedoff, while the hydrogen m-I_(D)Fs are maintained. Such effect enablesthe growth of mono-layer graphene in an open-atmosphere environment. Thehydrogen annealing technique can be utilized to create defects such asnanoscale pores and vacancies (e.g., at the atomic scale) in thegraphene layer(s). The D-peak-to-G-peak intensity (I_(D)/I_(G)) ratioincreases dramatically after hydrogen annealing when as-synthesizedgraphene on Cu exhibits an initial ratio of at least about 0.6. However,the I_(D)/I_(G) ratio does not change significantly after annealing ifthe initial ratio is lower than about 0.6. By controlling the annealingcondition, highly-defective graphene films with tunable defects aredirectly synthesized using a two-step flame method. Such defectivegraphene has a myriad of applications, such as ultrafiltering membranes,gas sensors, and optoelectronics and can be readily doped with metals(e.g., by using a solution of metal salts).

Using the modified multiple-inverse-diffusion-flame burner, monolayergraphene on copper substrate has been synthesized. As-received metalsubstrate with a native oxide layer can be pre-treated in a hydrogenreducing atmosphere. For example, the burner can be run initially withhydrogen as the sole fuel, which provides the hydrogen needed to reduceany residual oxide layer. Upon placing the copper foil/substratedownstream of the flames (e.g., for about 10 minutes), a hydrocarbon gasis added to the flow through the elevated/extended precursor-deliverylines. The copper foil/substrate is kept in the same position (e.g., forabout an additional 10 minutes) and then the hydrocarbon gas is removedfrom the flame (e.g., shut off). The copper foil/substrate is kept inthe same position (e.g., for about an additional 10 minutes) and thenremoved from above the hydrogen flame. Raman spectra was used to confirmthe presence of monolayer graphene.

A continuous rasterizing process based on the above procedure can beused to coat large area surfaces (see, e.g., FIG. 1B). Briefly, a firstburner reduces the oxide layer on the substrate surface; a second burnersynthesizes graphene on the surface; and a third burner anneals thegraphene to change its characteristics. A monitoring system (e.g., Ramanand Photoluminescence) may follow the burners to assess the coatingquality.

Using the modified multiple-inverse-diffusion-flame burner,nano-defective graphene on copper has also been directly synthesized.Upon placing the copper foil/substrate downstream of (e.g., above) theflames (e.g., for about 10 minutes), a hydrocarbon gas (e.g., methane)is added to the flow through the elevated/extended precursor-deliverylines. The copper foil/substrate is kept in the same position (e.g., forabout an additional 5 minutes), and then the hydrocarbon gas is removedfrom the flame (e.g., shut off). The foil is kept in the same position(e.g., for about an additional 0 to 10 minutes), and then removed fromabove the flame. Raman spectra can be used to confirm the presence ofnano-defective graphene and to identify the defective level for sampleswith different post-growth hydrogen annealing times. A high-resolutionTEM image shows the chemical structure of flame-synthesized defectivegraphene. Many kinds of graphene structural defects like vacancy, doublevacancy, and Stone-Wales defect can be clearly observed by comparing theenlarged TEM image of defective graphene and pristine flame-synthesizedmonolayer graphene at the same magnitude. UV-Visible Spectroscopy isused to determine the band-gap structure of graphene in the presence ofdefects. The band-gap energies can be measured via the DiffuseReflectance Derivative Peak Fitting (DPR) method, where the center ofeach Gaussian peak fit to the first derivative, dR∞/dλ, spectracorresponds to a band gap. The band energies of defective graphene inthe low-energy range (800 nm to 1000 nm) are observed. Graphene-basedultrafiltration membrane is fabricated by transferring as-synthesizednano-defective graphene from copper substrate to a standard precision5-μm round aperture. The estimated open-area diameter and percentagerespect to NDG with different I_(D)/I_(G) ratios are derived fromconductance tests.

The instant invention describes methods for the synthesis of monolayergraphene and/or nanodefective graphene. In a particular embodiment, themethods of the instant invention comprise the use of a modifiedmulti-element, inverse-diffusion flames (non-premixed) burner.Multi-element, inverse-diffusion flame burners are described, forexample, in WO 2012/116286, incorporated by reference herein. The burnercomprises an array of tiny stabilized flames that form a uniformflat-flame front, with respect to substrates placed well downstream.Each of the diffusion flames is run in the inverse mode(“under-ventilated”). Generally, for each tiny stabilized flame of theburner, the oxidizer (e.g., air, oxygen, etc.) is provided by a centerfeed tube and the fuel (e.g., hydrogen or a hydrocarbon precursor suchas methane, ethylene, acetylene, etc.) is provided by tubes whichsurround it. The burner is capable of continuous operation in open andclosed environments. The burner may be operated in an ambient-airenvironment or an inert environment (e.g., an inert gas such as N₂, Ar,He).

In a particular embodiment, the multi-element, inverse-diffusion flamesburner of the instant invention is modified such that the precursordelivery tubes are extended beyond (e.g., above) the main burnersurface. For example, the precursor delivery tubes are about 1 mm toabout 10 mm above the main burner surface, particularly about 2 mm toabout 8 mm; about 3 mm to about 7 mm; about 4 mm to about 6 mm; or about5 mm above the main burner surface. FIGS. 1 and 2 depict the modifiedmulti-element, inverse-diffusion flames burners of the instantinvention.

The instant invention encompasses the synthesis of carbon-based,particularly graphene, materials or structures (e.g., nanostructures,microstructures, surface structures of materials). For example, thestructure may be a particle, granule, fiber, wire, preform, composite,polymer, film, disc, plate, sheet, or flake. In a particular embodiment,the structure is a film, disc, plate, sheet, or flake. In a particularembodiment, the structure is a graphene sheet. The structures may bemulti-layered, bi-layered, or mono-layered.

In a particular embodiment, the methods of the instant invention involvedepositing pyrolyzed species onto a heated substrate to generatecarbon-based materials (e.g., nanostructures) such as graphene sheets.As explained above, the stabilized flame of the multiple-flame burnermay comprise an oxidizer-feed tube in the center and fuel-feed tubessurrounding it. In a particular embodiment, the oxidizer is air or O₂.The O₂ can be diluted to any desired concentration with inert gases(e.g., N₂). In a particular embodiment, the fuel is hydrogen or ahydrocarbon precursor such as methane, ethylene, acetylene, or mixturesthereof. In a particular embodiment, the modified multi-element,inverse-diffusion flames burner also comprises hydrogen-only feed tubes.

While the structures of the instant invention are typically referred toas graphene, the structures may also be doped or alloyed. For example,methods of synthesizing graphene with nitrogen and/or boron, as well asother elements, is also encompassed by the instant invention. Forexample, ammonia (NH₃) can be introduced with the hydrocarbon fuel(e.g., CH₄) to provide a source of nitrogen, such that the NHx speciesformed during flame decomposition are incorporated directly into thegraphene structure. Similarly, borane (BH₃) or borane-ammonia (H₃NBH₃)can be introduced with the hydrocarbon fuel to provide a source of boronand/or nitrogen. It is possible that the hexagonal symmetry of thegraphene can be retained, while incorporating boron or boron nitrideinto the structure. Additionally, halogenation can be applied toincorporate fluorine and other elements into the structure using theappropriate precursors. Properties of the nanostructures may be altered.In addition, the defective or highly-defective graphene can also bedoped by “filtering” a metal ion solution such that the metal ions fillin the nanopores or atomic vacancies.

The substrate upon which the pyrolyzed species are deposited is solidsubstrate, particularly a metal substrate. Examples of substratesinclude, without limitation: copper, nickel, steel, transition metals,alloys thereof, and spinels thereof In a particular embodiment, thesubstrate is copper. The substrate may be ultrapure (e.g., ≥99.999%pure, or ≥99.9999% pure). The substrate may be less than ultrapure(e.g., ≥98% pure, ≥99% pure, ≥99.5% pure, ≥99.7% pure, ≥99.8% pure, or≥99.9% pure, while being less than ultrapure). When defective grapheneis being synthesized, the substrate is less than ultrapure. In aparticular embodiment, the substrate is Cu which is about 99% to about99.9% pure.

The substrates themselves can translate and/or rotate effortlessly in acontinuous production mode. In one embodiment, the pyrolysis vapors inthe hot gas stream uniformly coat flat substrates with nanostructures(e.g., films). The reactor may direct the gas stream onto a moving belt,thereby enabling continuous fabrication. The burner(s) may also bemoved, oriented, and/or rasterized across a surface. Indeed, operationof a burner of the instant invention has no scaling problems by allowingfor stability at all burner diameters, where the issuing flow velocitycan be independent of the burner diameter. The post-flame gasesdownstream are quasi 1-D in that they are radially-uniform intemperature and chemical species. Thus, larger burners and substratescan be used, while ensuring uniform growth rates at the substrate.Additionally, the substrates can be placed on a conveyor-belt forhigh-production throughput or the burner can be translated or rasterizedto generate a very large area coating or deposit. Rotating thesubstrates (e.g., in spin coating) to maintain flatness and uniformityof the growth layers can also be performed. Further, for productionpurposes, scalability can be readily accomplished by laying side-by-sideseveral (e.g., identical) honeycomb structures (square or hexagonshaped) to form any desired burner size, shape or form.

In a particular embodiment, the methods of the instant inventionencompass pre-treating the substrate to remove any oxide layer. Forexample, a copper substrate may be pre-treated prior to graphene growth.The unwanted oxide layer on the metal substrate may be removed viahydrogen reduction by using only hydrogen as fuel (e.g., >1 globalequivalence ratio) in the modified multiple inverse-diffusion flameburner. After pre-treatment, a hydrocarbon precursor is added to growthe graphene in a continuous manner. In a particular embodiment, thesubstrate is pre-treated with only hydrogen as fuel in the burner forabout 1 second to about 30 minutes, particularly about 1 minute to about20 minutes, about 2 minutes to about 15 minutes, about 5 minutes toabout 15 minutes, or about 10 minutes.

In accordance with the instant invention, methods for synthesizingmonolayer graphene are provided. In a particular embodiment, the methodcomprises: 1) reducing a metal substrate (e.g., Cu) in a modified,multiple inverse-diffusion flame burner wherein hydrogen (H₂) is used asthe only fuel; 2) adding a hydrocarbon precursor through the fuel lines(the modified (elevated) fuel lines) of the modified, multipleinverse-diffusion flame burner to synthesize the graphene; and 3)annealing the flame-synthesized graphene by running the modified,multiple inverse-diffusion flame burner wherein hydrogen (H₂) is used asthe only fuel (i.e., the flow of hydrocarbon precursor is stopped). In aparticular embodiment, an inert gas (e.g., Ar) is present during step 1,2, and/or 3. In a particular embodiment, Raman spectroscopy (e.g.,online) is performed to confirm the presence of monolayer graphene. In aparticular embodiment, the method further comprises transferring (e.g.,via wet chemistry method) the monolayer graphene to another substrate.

In a particular embodiment, the temperature of the modified, multipleinverse-diffusion flame burner is greater than about 500° C.,particularly greater than about 850° C., greater than about 900° C.,greater than about 950° C., greater than about 1000° C., or is about1000° C. In a particular embodiment, the temperature of the modified,multiple inverse-diffusion flame burner is about 950° C. to about 1050°C. or about 975° C. to about 1025° C. The temperature of the burnershould remain below the melting temperature of the substrate (e.g.,˜1085° C. for copper).

In a particular embodiment, the pre-treatment of the substrate with onlyhydrogen as fuel in the burner (step 1) is for about 1 second to about30 minutes, particularly about 1 minute to about 20 minutes, about 2minutes to about 15 minutes, about 5 minutes to about 15 minutes, orabout 10 minutes.

In a particular embodiment, step 2 comprises using hydrogen and ahydrocarbon precursor (e.g., CH₄) as fuel. The input ratio of thehydrocarbon precursor and hydrogen can be from about 1:1 to about1:1000, particularly about 1:25 to about 1:250, about 1:50 to about1:200, or about 1:100. Step 2 can be performed for as long as graphenegrowth is desired (particularly for at least about a minute or for atleast 5 minutes).

In a particular embodiment, the graphene annealing with only hydrogen asfuel in the burner (step 3) is for about 1 second to about 30 minutes,particularly about 1 minute to about 20 minutes, about 2 minutes toabout 15 minutes, about 5 minutes to about 15 minutes, or about 10minutes. The annealing step may be performed until the I_(G)/I_(2D)ratio of the graphene is reduced to less than about 1.0, less than about0.9, less than about 0.8, less than about 0.7, less than about 0.6, lessthan about 0.5, or less than about 0.4.

Using the modified multiple-inverse-diffusion-flame burner,nano-defective graphene on copper substrate has also been directlysynthesized. Upon placing the copper foil/substrate downstream of theflames (e.g., for about 10 minutes), a hydrocarbon gas is added to theflow through the elevated/staged precursor-delivery lines. The copperfoil/substrate is kept in the same position (e.g., for about anadditional 5 minutes), and then the hydrocarbon gas is removed from theflame (e.g., shut off). The foil is kept in the same position (e.g., forabout an additional 0 to 10 minutes), and then removed. Raman spectra isused to confirm the presence of nano-defective graphene and to identifythe defective level for samples with different post-growth hydrogenannealing time.

In accordance with the instant invention, methods for synthesizingnano-defective graphene (also referred to as nanoporous graphene) areprovided. Nano-defective graphene is graphene with atomic-scale defects(e.g., vacancies, Stone-Wales defect) and/or nano-scale defects (e.g.,grain boundaries, overlaps) and pores. In a particular embodiment, themethod comprises: 1) reducing a metal substrate (e.g., Cu) in amodified, multiple inverse-diffusion flame burner wherein hydrogen (H₂)is used as the only fuel; 2) adding a hydrocarbon precursor through thefuel lines (the modified (elevated) fuel lines) of the modified,multiple inverse-diffusion flame burner to synthesize the graphene; and,optionally, 3) annealing the flame-synthesized graphene by running themodified, multiple inverse-diffusion flame burner wherein hydrogen (H₂)is used as the only fuel (i.e., the flow of hydrocarbon precursor isstopped). In a particular embodiment, an inert gas (e.g., Ar) is presentduring step 1, 2, and/or 3. In a particular embodiment, Ramanspectroscopy is performed to confirm the presence of nano-defectivegraphene. In a particular embodiment, the method further comprisestransferring (e.g., via wet chemistry method) the nano-defectivegraphene to another substrate.

The nano-defective graphene of the invention may exhibit band gaps.Further, the properties of the nano-defective graphene are readilytunable with the methods of the instant invention (see Examples below).For example, the degree and/or amounts of defects and/or the band gapscan be modulated by varying the conditions (e.g., the length of thevarious steps) of the method. The resultant nano-defective graphene maybe used for ultrafiltration (e.g., water desalination, ion-selectivemembranes, gas separation membranes, and gas molecule sensors).Additionally, the nano-defective graphene, with its atomic-scalevacancies, can be readily doped (e.g., with atoms or molecules forenhanced performance for various applications such as optoelectronicsand catalysis).

In a particular embodiment, the temperature of the modified, multipleinverse-diffusion flame burner is greater than about 850° C., greaterthan about 900° C., greater than about 950° C., greater than about 1000°C., or is about 1000° C. In a particular embodiment, the temperature ofthe modified, multiple inverse-diffusion flame burner is about 950° C.to about 1050° C. or about 975° C. to about 1025° C. The temperature ofthe burner should remain below the melting temperature of the substrate(e.g., ˜1085° C. for copper).

In a particular embodiment, the pre-treatment of the substrate with onlyhydrogen as fuel in the burner (step 1) is for about 1 second to about30 minutes, particularly about 1 minute to about 20 minutes, about 2minutes to about 15 minutes, about 5 minutes to about 15 minutes, orabout 10 minutes.

In a particular embodiment, step 2 comprises using hydrogen and ahydrocarbon precursor (e.g., CH₄) as fuel. The input ratio of thehydrocarbon precursor and hydrogen can be from about 1:1 to about1:1000, particularly about 1:25 to about 1:250, about 1:50 to about1:200, or about 1:100. Step 2 can be performed for as long as graphenegrowth is desired (particularly for at least about a minute or for atleast 5 minutes). In a particular embodiment, the substrate is less thanultrapure (e.g., less than 99.99% pure or less than 99.9% pure).

As stated hereinabove, step 3 in the method is optional. The grapheneshould have an I_(D)/I_(G) ratio of about 0.6 or higher prior toperforming step 3. In a particular embodiment, the graphene annealingwith only hydrogen as fuel in the burner (step 3) is for about 1 secondto about 30 minutes, particularly about 1 minute to about 20 minutes,about 2 minutes to about 15 minutes, about 5 minutes to about 15minutes, or about 10 minutes. The annealing step may be performed untilthe I_(D)/I_(G) ratio of the graphene is increased to a desired amount(e.g., greater than about 0.8, greater than about 1.0, or greater thanabout 1.2).

The following examples describe illustrative methods of practicing theinstant invention and are not intended to limit the scope of theinvention in any way.

EXAMPLE 1 Modified Multi Element Inverse-Diffusion Flame Burner

A schematic of the modified multi-element inverse-diffusion flame setupis shown in FIG. 1. Each distinct flame in the planar array of the mainburner surface runs in an inverse mode (“under-ventilated”). Distinctprecursor delivery tubes are staged above/beyond the main burner surfaceat a fixed height of 5 mm in order to deliver hydrocarbon precursor(e.g., methane, ethylene, acetylene, etc.) to the post-flame regiondirectly. This design prevents hydrocarbon precursor from fullyoxidizing/decomposing by bypassing the multiple flames. At the burnerbase, oxidizer (e.g., air, oxygen, or other oxidizing agent (e.g., O₃,F₂, Cl₂, etc.) is delivered through small individual oxidizer tubes.Both precursor and oxidizer tubes are made of stainless steel andmounted through a stainless steel honeycomb fixture. Fuel (e.g.,hydrogen, methane, ethylene, acetylene, etc.) is delivered through theempty channels of the honeycomb fixture. Water cooling is achieved byuse of copper coil wrapped around the burner. A quartz cylinderencompassing the flame and post-flame regions prevents air permeationfrom the ambient and reduces convection heat losses. The quartz cylinderis open at the downstream end, after a certain length, exhausting to ahood. Therefore, the setup is open to atmospheric condition.

All the gas flow rates were regulated by mass flow controllers (MFCs). ALabVIEW program on a PC was used to control the MFCs for convenient andprecise flow delivery of multiple gases, reducing the experimental errorfrom the control side, which ensures the reproducibility of the results.Excess fuel and precursor are consumed in an after-burner mounteddownstream of the open end of the quartz cylinder before exhausting intoa hood. The substrate is mounted to a rod and inserted into thepost-flame flow field from the open end of the quartz cylinder. The rodhas threads that enable adjustment of the substrate height with respectto the burner base. A sidewall slot is machined into the quartz cylinderto allow access for an igniter and thermocouple.

Numerical Simulation to Design Modified Burner

A flame simulation was utilized using ANSYS Fluent to design the heightof precursor tubes prior to modifying the m-I_(D)F burner. Gambit wasused to define the geometry and mesh, which comprises four individualflames. Both two-step laminar reaction for methane/air based onArrhenius kinetics and GRI-Mech 1.2 were used to model the flames andguide construction of the final burner. Previous experimental data serveas flow rate inputs in the simulation. Employing adiabatic radialboundary conditions, the results of using the two-step laminar reactionmechanism was determined. The temperatures from the individual flamesmerge at ˜5 mm above the tubing exits (the length of the tubes is 10mm), and the radial boundary has a constant temperature ˜1600 K afterflames merge. A simulation of constant temperature radial boundarycondition at 1600 K using detailed chemical kinetics, i.e., GRI-Mech 1.2mechanism, was then performed. The individual flame temperatures mergein a shorter distance than they do in using the two-step mechanism.Based on the simulation results, the m-I_(D)F burner design was modifiedwith distinct precursor tubes elevated above the burner surface at afixed height of 5 mm.

Temperature Measurement

Gas-phase temperatures were measured using a 125 μm Pt/Pt −10% Rhthermocouple (Stype, OMEGA, Model: P10R-005). A silica coating wasapplied to the thermocouple junction to prevent catalytic oxidation onthe Pt-based thermocouple. The coating was performed using a smallco-flow burner, where silicone oil was injected using a syringe pump.The coating uniformity (3±0.5 μm) was confirmed under a microscope. Thethermocouple was held for 2 seconds within the flame, and the procedurewas repeated multiple times to minimize error.

Sample Preparation

Substrates used for these experiments consist of various metals andnon-metals including copper foil and plate, nickel foil, stainless steelfoil, silicon wafer and several others. All substrates were cut into 1.5cm×1.5 cm squares and placed above the burner, but within theencompassing quartz cylinder. The substrate can be positioned parallelto the flame flow, or perpendicular to the flow, or tilted an angle withthe flow. In all cases, no prior substrate preparation was performed. Alist of the substrate materials is shown in Table 1.

TABLE 1 List of substrates. Material Purity Thickness Company Part #Copper Foil   99.8% 0.025 mm Alfa Aesar 13382 Puratronic Copper Foil99.9999% 0.25 mm Alfa Aesar 42974 Puratronic Copper Foil 99.9999% 0.1 mmAlfa Aesar 42973 Nickel Foil    99% 0.025 mm Alfa Aesar 12722 SiliconWafer (1 0 0) *N-Phosphorus 381 microns El-Cat 30 Silicon Wafer (1 0 0)*P-Boron 500 microns + University 43 300 nm oxide Wafer Stainless Steel304 Foil N/A 0.025 mm Alfa Aesar 41580 Stainless Steel 304L Foil N/A0.15 mm Goodfellow FA140230 Stainless Steel 316L Foil N/A 0.05 mmGoodfellow FF210250

In-Situ Raman Spectroscopy of Gas-Phase Species in the Flow Field

The second harmonic (532 nm) of an injection-seeded Nd:YAG laser(Quanta-Ray LAB 170) operating at 10 Hz (˜9 ns FWHM) was utilized forin-situ Raman measurement of gas-phase species. The laser beam wasfocused by a 500-mm focal-length plano-convex fused-silica lens to aprobe volume with a waist diameter of approximately 200 μm. Aspectrometer (Acton SpectraPro-500i) with a 600 groove/mm grating and anICCD camera (Princeton Instruments, PIMAX 1300HQ-25-FO) were used forimaging. The slit of the spectrometer was 200 μm. An oscilloscope(Agilent Infinium 54845A, 1.5 GHz sampling rate) was employed to monitorthe timing of the laser pulse and camera gating. The camera gate widthtime was set to 20 ns to reduce interference and background. The burnerwas mounted on a 2-D translator in order to move in x and z directions.The emissions were collected at 90° by a 400-mm focal-length achromaticlens, then passed through a Raman holographic notch filter (KaiserHSPF-532.0-2.0), and finally focused by a 300-mm focal-length achromaticlens onto the slit of a 0.5 m imaging spectrometer.

Ex Situ Raman Spectroscopy of As-Synthesized Nanomaterials

Raman scattering is the inelastic scattering of an incident photon uponinteraction with an atom or molecule. When incidents photons arescattered from matter, most of them are elastically scattered (Rayleighscattering) at the same frequency as the incident photons. If thescattered photons have lower frequencies than that of incident photons,the scattering is called Stokes scattering, giving the Stokes lines inthe Raman scattering spectrum. To the contrary, if the scattered photonshave higher frequencies, the scattering is called Anti-Stokesscattering, giving the Anti-Stokes lines.

Raman spectroscopy is a widespread spectroscopic technique used toobserve vibrational and rotational modes within a system. It has beenextensively used in characterizing various carbon systems (e.g.,amorphous carbon, metallic and semiconducting SWNTs, graphite, etc.)with only a few prominent features including a couple of very intensebands in the 1000-2000 cm⁻¹ range and few other second-order modulations(Ferrari, A. C. (2007) Solid State Commun., 143(1-2):47-57). In theRaman spectra of carbon, the main bands are called G and D peaks, whichare located at around 1560 and 1360 cm⁻¹, respectively, for visibleexcitation. The G peak is related to the bond stretching of all pairs ofsp² atoms in both rings and chains, and the D peak is related to thebreathing modes of sp² atoms in rings (Ferrari, A. C. (2007) Solid StateCommun., 143(1-2):47-57). All sp² carbon materials normally present astrong peak called G′ (or 2D) peak in the range 2500-2800 cm⁻¹. Formonolayer graphene, the G peak appears at 1582 cm⁻¹, and the G′ peak atabout 2700 cm⁻¹, using 514 nm excitation (Malard, et al. (2009) PhysicsRep., 473(5-6):51-87). At the edge of a graphene sample or in the caseof a disordered sample, a disordered-induced D peak manifests at around1350 cm⁻¹ (Malard, et al. (2009) Physics Rep., 473(5-6):51-87). Indefective graphite, there is a so-called D′ peak (at ˜1620 cm⁻¹)resulting from the double resonance process (Malard, et al. (2009)Physics Rep., 473(5-6):51-87). The ratio between G and G′ peaks can beused to estimate the number of graphene layers and the ratio between Dand G peaks can be used to measure the disorder induced from edges,impurities, domain boundaries, wrinkles, etc.

Transmission Electron Microscopy

A beam of accelerated electrons is transmitted through an ultra-thinspecimen in a transmission electron microscope (TEM). The waveinteractions of the electrons transmitted through the specimen givesignificantly higher-resolution information compared with opticalmicroscopes since the De Broglie wavelength of electrons is many ordersof magnitude smaller than that for visible light. TEM is widely used forimaging crystal structures of nanomaterials. Another main function ofTEM is selected area electron diffraction (SAED) that can be used todetermine the crystallinity of samples. Herein, a high-resolution TEM(HRTEM; JEOL 2010F) was used to characterize graphene samples.

Atomic Force Microscopy

Atomic-force microscopy (AFM) is a type of scanning probe microscopywith nanoscale resolution. A cantilever with a sharp probe at its end isused to scan the surface depth profile of the samples. The AFM can beoperated in either static (contact) mode or dynamic (tapping ornon-contact) mode for a number of applications. In static mode, wherethe probe tip is dragged across the specimen surface, a firm contactwith the solid surface is required. In tapping modes, short-range forcesare detected by oscillating the cantilever probe tip close enough to thesample surface without contact. Tapping mode—which prevents the tip fromsticking to or damaging the surface—is more suitable for thin-layermaterials like graphene. AFM (Bruker Dimension FastScan) was employed todetermine the morphology and uniformity of graphene samples on a siliconwafer (surface roughness ˜1 nm).

EXAMPLE 2

The breakdown of the self-limiting mechanism makes it difficult todeposit monolayer graphene (MLG) on Cu at atmospheric pressure. However,by reducing the methane concentration in the gas mixture, MLG has beenachieved in atmospheric-pressure chemical vapor deposition (APCVD)(Bhaviripudi, et al. (2010) Nano Lett., 10(10):4128-4133). The effect ofmethane flow rate and methane partial pressure has been studied ongraphene growth rate and domain size (Li et al. (2010) Nano Lett.,10(11):4328-4334). Lower flow rate and partial pressure of methane leadto lower growth rate and less nucleation density of graphene, which areessential for the growth of large-crystal monolayer films. Graphenecrystals grown from different nucleation sites with differentorientations can only coalesce into polycrystalline films (Wang et al.(2012) J. Am. Chem. Soc., 134, no. 8, pp. 3627-3630). In flamesynthesis, the total flow rate and carbon flux are much higher than thatin APCVD and low-pressure chemical vapor deposition (LPCVD), in order tostabilize the flames, which can lead to smaller graphene domains andadlayers. That is why few-layer graphene (FLG) grown in open-atmosphereflame synthesis exhibits a higher D-peak to G-peak ratio (I_(D)/I_(G))ratio and sheet resistance than that for CVD-grown graphene (Memon etal. (2011) Carbon 49(15):5064-5070). Moreover, the activation energy ofgraphene nucleation in atmospheric pressure (9 eV) is substantiallyhigher than that in low pressure (4 eV) (Vlassiouk, et al. (2013) J.Phys. Chem. C, 117(37):18919-18926).

Various parameters have been studied for flame synthesis of graphene.After optimization of synthesis conditions, including substratematerial, temperature, flow rate, and growth time, bilayer graphene(BLG) was produced for the first time using an open-atmosphere flamesynthesis method. However, by further lowering methane concentration tothe condition of APCVD, MLG was not observed for the cases examined. Thehigh flow flux and large numbers of combustion products (e.g., H₂O, OH,CO, CO₂) seem to make it difficult to directly translate conditions forCVD to that for flame synthesis.

Hydrogen plays a vital role in graphene growth as an activator ofsurface-bound carbon and an etching reagent for the “weak” carbon-carbonbonds that controls the graphene domains (Vlassiouk, et al. (2011) ACSNano, 5(7): 6069-6076). The minimum temperature for effective hydrogenetching is 850° C. (Vlassiouk, et al. (2011) ACS Nano, 5(7): 6069-6076).For the flame synthesis of graphene, the role of hydrogen has beenmainly investigated for the substrate pretreatment stage and the growthstage (Memon, et al. (2013) Proc. Combustion Inst., 34(2):2163-2170). Apost-growth hydrogen thermal etching on APCVD-synthesized graphene hasbeen performed at 1000° C. for 1.5 minutes to obtain MLG (Yao, et al.(2012) Carbon 50(14):5203-5209). Moreover, a high-temperature thermalannealing process was found to be effective for curing the defects ingraphene, calling it a “self-healing” mechanism (Chen, et al. (2013)Applied Phys. Lett., 102(10):103-107). Herein, hydrogen annealing isemployed to change the quality of flame-synthesized graphene.

Experiment

The synthesis setup used herein is a modified multi-elementinverse-diffusion flames (m-I_(D)Fs) burner. The experimental processcan comprise two operations during a single experiment using the samemodified m-I_(D)Fs burner: 1) flame synthesis of graphene, 2) hydrogenannealing of flame-synthesized graphene (see FIG. 2). Hydrogen annealingwas performed on flame-synthesized BLG on Cu.

Prior to synthesis, the Cu substrate was reduced in the hydrogenenvironment at 1000° C. for 10 minutes, using the modified m-I_(D)Fsburner running only hydrogen as fuel with no flow through the extendedprecursor tubes, to remove any oxide layer and enlarge the grain size.Then methane was introduced into post-flame gases via the extendedprecursor delivery tubes to initiate graphene growth on Cu. The growthtemperature was maintained at 1000° C., and the input ratio between CH₄and H₂ was 1:100, where hydrogen serves as the fuel emanating from thebase of the modified m-I_(D)Fs burner. After 5 minutes of growth,methane supply through the precursor tubes was shut off, but hydrogencontinued to flow sustaining the m-I_(D)Fs at the base of the burner. Assuch, the substrate experiences annealing in a hydrogen (and argoninert) environment at 1000° C. for 10 minutes. After annealing in thehydrogen-rich environment produced by the underventilated multipleflames at the base of the burner, the m-I_(D)Fs were extinguished simplyby shutting down the oxidizer flow.

Finally, the substrate was cooled down to room temperature with acontinuous flux of inert argon gases.

As-synthesized graphene was characterized by Raman spectroscopy(Renishaw 1000, 514 nm laser wavelength, 50x magnitude). Transmissionelectron microscopy (TEM, JOEL 2010 F, 200 kV) was employed to study thecrystal structure and morphology of graphene samples. TEM sample wasprepared by transferring graphene to a lacey TEM grid by following thewet-chemistry process. Atomic-force microscopy (Dimension FastScan,Bruker Nano) with probes (Fastscan A, 5 nm tip radius) was used fortapping mode scanning and imaging of the graphene samples on Si/SiO₂.

Graphene deposited on copper can be transferred onto other substratesfor characterization or fabrication of devices. The common transferprocess of graphene is by using wet-chemistry method. As-grown grapheneon Cu is first spin-coated with a poly(methyl methacrylate) (PMMA)protective coating. After the PMMA coating is cured, the Cu substrate isetched away by using iron nitride or iron chloride aqueous solution. ThePMMA/Graphene stack is washed in deionized (DI) water and transferred tothe target substrate. The last step is to remove the PMMA coating withacetone. However, the traditional process can cause the graphene to formcracks because of intrinsic mechanical properties of monolayer graphene.Before removing the initial PMMA layer, redepositing another layer ofPMMA can reduce the cracks of graphene after the transfer process (Li etal. (2009) Nano Lett., 9(12):4359-4363). Transfer to an insulatingsurface (e.g., silicon or quartz) is required to measure optical andelectronic properties of synthesized graphene. Silicon wafer is a commonsupport material for graphene-based semiconductor applications.

Results

Typical Raman spectra of graphene sample before and after hydrogenannealing are shown in FIG. 3. For a 10-minute post-growth treatment at1000° C., the overall I_(G)/I_(2D) ratio is significantly reduced from˜1 to ˜0.5, which means a BLG is converted into a MLG. This resultdemonstrates that hydrogen reduces the number of graphene layers byeffectively etching additional layers and growth fronts throughhydrogen's interactions with “weak” carbon-carbon bonds and danglingbonds. The D-peak to G-peak intensity (I_(D)/I_(G)) ratio is nearlyconstant before and after annealing. The “self-healing effect” has notbeen observed in this case. The self-healing of defective graphene withnanoscale vacancies induced by an argon plasma was observed when thermalannealing samples in an argon atmosphere (Chen, et al. (2013) Appl.Phys. Lett., 102(10):103-107). The curing of the vacancies can be due tothe mobility and rearrangement of carbon atoms on the Cu surface at hightemperature. There are two possible reasons why the D peak intensity isnot reduced after hydrogen annealing. First, the D peak inflame-synthesized graphene is mainly due to the submicron domain size(Memon, et al. (2011) Carbon 49(15):5064-5070). Since the I_(D)/IG ratiois inversely proportional to the domain size of graphene (Cancado, etal. (2011) Nano Lett., 11(8):3190-3196; Lucchese, et al. (2010) Carbon48(5):1592-1597), the ratio should stay the same if the domain size isunchanged. Second, the “self-healing” effect is offset by the hydrogenetching effect. Based on the Raman spectra, the inference may be drawnthat by using hydrogen annealing, the adlayer graphene can beeffectively etched away without introducing new defects or reducingdomain size in flame-synthesized graphene.

Typical Raman spectra of flame-synthesized MLG features a D peak locatedat 1346 cm⁻¹, a G peak at 1585 cm⁻¹, and a 2D peak centered at 2695 cm⁻¹(see FIG. 3). All peaks can be fitted by a Lorentzian profile. Thesymmetric 2D peak has a narrow full width at half maximum (FWHM) of ˜35cm⁻¹. For high-quality MLG synthesized in CVD, the I_(G)/I_(2D) ratio isreported in the range from ˜0.4 to ˜0.5, and the FWHM of the 2D peak isbetween 30 and 40 cm⁻¹ (Li, et al. (2011) J. Am. Chem. Soc.,133(9):2816-2819; Hu, et al. (2012) Carbon 50(1):57-65). The presence ofa defect-induced D peak indicates the existence of subdomain boundariesand multi-layer graphene formation (Reina, et al. (2009) Nano Lett.,9(1):30-35; Hu, et al. (2012) Carbon 50(1):57-65). The I_(D)/I_(G) ratiois usually lower than 0.05 for CVD-synthesized graphene. However, theI_(D)/I_(G) ratio of large-area FLG has been reported to be between 0.05and 0.3 (Memon, et al. (2011) Carbon 49(15):5064-5070). Here, theflame-synthesized graphene exhibits a I_(D)/I_(G) ratio of 0.4. From theenlarged TEM image, the hexagonal arrangement of carbon atoms ingraphene can be observed (FIG. 4). The selected area electrondiffraction (SAED) pattern indicates that the MLG is not a perfectsingle crystal and probably has adlayers. The TEM images at differentresolutions show a polycrystalline graphene film with clear grainboundaries (FIG. 4). Considering both the Raman spectra and TEM results,the MLG sample possesses additional layers, submicron domains, and othertypes of defects like vacancies and nanopores. AFM image of MLG onSi/SiO₂ confirms the uniformity of the film at the micrometer scale. Thehighlighted spots in the image are the residues stuck in the film duringthe wet-chemistry transfer process. A depth-profile scanning has beenperformed at different edges, but there is no conclusive result for thefilm thickness. The theoretic thickness of MLG is ˜0.34 nm, but thesurface roughness of Si/SiO₂ is measured to be ˜1 nm, which createssignificant variance in the thickness measurement.

Single-crystal MLG displays remarkable electro mobility at roomtemperature. However, the absence of a bandgap in perfect MLG preventsit from semiconductor applications. A tunable bandgap has been observedin BLG, and the gap can be tuned by an external electrostatic potential(Castro, et al. (2007) Phys. Rev. Lett., 99(21):216802; Zhang, et al.(2009) Nature 459(7248):820-823; Ohta, et al. (2006) Science313(5789):951-954; McCann, et al. (2006) Phys. Rev. Lett., 96(8):086805). Graphene defects such as vacancies and heteroatoms can open upa bandgap in MLG (Guo et al. (2011) Insciences J., 1(2):80-89; Cretu, etal. (2010) Phys. Rev. Lett., 105(19):196102; Yuan, et al. (2014)Materials Today, 17(2):77-85). Flame-synthesized MLG naturally exhibitsa higher defective level, implying the existence of a small-percentageof BLG, vacancies, nano-scaled pores, and submicron domains. Theseimperfections induce a bandgap in flame-synthesized MLG for variousapplications.

A hydrogen etching effect on flame-synthesized graphene was discoveredduring a post-graphene-growth hydrogen annealing process. With thisprocess, high-quality (meaning minimal defects) bilayer graphene can betailored towards monolayer graphene (MLG). Such technique enables thesynthesis of MLG using open-atmosphere flame synthesis for the firsttime. The production of MLG at atmospheric pressure is considered achallenge because the “self-limiting” mechanism of Cu no longer holds atelevated pressure. Even though MLG has been achieved inatmospheric-pressure (but confined) CVD by carefully controlling thepartial pressure and flow rate of the precursor, it is still verychallenging to grow MLG using any gas-phase CVD synthesis method in anopen unconfined environment. Hydrogen atoms are found to be effectiveetching agents for weak carbon bonds and dangling bonds in adlayers ofgraphene. Such etching process does not damage the fine crystals ofgraphene, and no obvious shrinkage in graphene domain size is observedbased on Raman spectra. Unlike CVD processes, this method is unconfinedand more suitable for the continuous large-scale production of MLG overlarge surfaces at reduced costs.

EXAMPLE 3

Graphene's defects can be categorized based on the scale. Atomic-scaledefects include vacancy-type defects (reconstructed point defects),hetero-atoms and Stone-Wales defects. The vacancy-type defects can becreated by electron irradiation in graphene, where foreign species canbe trapped (Cretu et al. (2010) Phys. Rev. Lett., 105(19):196102). Bypurposefully introducing such defects, it is possible to open up abandgap in graphene for semiconductors. Submicron-scale defects ingraphene such as pores and grain boundaries afford many potentialapplications in membranes and sensors.

Nanoporous graphene (NPG) has been extensively studied in recent years.Through the formation of nano-scale pores in a large area graphenesheet, it is possible to open an energy band gap for application infield effect transistors (FETs) (Yuan, et al. (2014) Materials Today17(2):77-85). Creating pores within graphene can also increase theamount of edges that act as adsorbing sites for gas molecules sensing.Moreover, NPG can be fabricated to be effective separation membranes forion selection and water desalination (Surwade, et al. (2015) Nat. Nano,10(5):459-464). Various methods have been employed to create nano-scalepores in graphene films, including focused electron beam irradiation,nanoimprint lithography, photocatalytic oxidation, and catalytichydrogenation (Yuan, et al. (2014) Materials Today 17(2):77-85). For thefabrication of ion-selective graphene membranes, pristine graphene canbe exposed to ion bombardment and oxidative etching. The pore size canthen be tuned by controlling the exposure time. Raman spectra are usedto measure the defects. The I_(D)/I_(G) ratio can conveniently indicatethe defective level in the graphene film. However, Raman spectra cannotprovide comprehensive information about what types of defects exist.Further TEM analysis is then needed. For a graphene sample withI_(D)/I_(G) ratio ˜1, the pore density is on the order of 1 pore/100nm².

In general, the current technique for creating defects (e.g., vacancies,pores) in graphene is by damaging CVD-grown graphene films usingelectron or ion beam, which requires an additional setup for graphenemodification after synthesis. A direct synthesis method for defectivegraphene would be favorable for many applications. Herein, the etchingeffect of hydrogen annealing was demonstrated, being able to tailor thenumber of graphene layers down to 2 or 1. The influence of hydrogenannealing on graphene samples with different original conditions andsubstrate materials were studied using the modified multi-elementinverse-diffusion flame setup. A direct synthesis method to producehighly-defective graphene was demonstrated. Furthermore, properties ofas-synthesized highly-defective graphene were investigated.

Experiment Hydrogen annealing was examined on substrates includingcommercial Ni and Cu foils (Alfa Aesar), ultrasmooth Cu provided by ArmyResearch Laboratory, and Si/SiO₂ wafer (University Wafer). UltrasmoothCu substrate is electropolished on the desired area with the rest of theCu surface roughness similar to commercial Cu (Griep et al. (2016) NanoLett., 16(3):1657-1662). Such electropolishing process can reduce the Cu(Alfa Aesar, 25 μm, 99.8%) surface roughness by over 90%. An enhancementin graphene mechanical properties has been observed by using ultrasmoothCu as a substrate in CVD (Griep et al. (2016) Nano Lett.,16(3):1657-1662). Graphene films were synthesized on metal substratesusing the modified m-IDF setup, but graphene samples on Si/SiO₂ waferswere prepared by two other approaches. One was using pulse-laserdeposition (PLD) to put graphene on Si/SiO₂. The PLD system uses aNd-YAG Q-switched laser (532 nm and 266 nm) as the energy source andpyrolytic graphite as a target to deposit FLG on Si/SiO₂ at 900° C. in ahigh vacuum chamber (10⁻⁶ torr). Another approach was transferringflame-synthesized graphene from Cu to Si/SiO₂ using the wet-chemistrymethod. Hydrogen annealing temperature was maintained at 1000° C. forall cases.

Raman spectroscopy (Renishaw 1000, 514 nm laser wavelength,50×magnitude) was employed as the main tool to characterize the qualitychange of graphene before and after hydrogen annealing. The I_(G)/I_(2D)ratio was used to identify the change in the number of layers ofgraphene. The I_(D)/I_(G) ratio gives a qualitative indication ofgraphene domain size and number of defects. Room temperatureScanning/Transmission Electron Microscope (FEI Talos F200X S/TEM, 200kV) was used to investigate the morphology of the nanoscale defectswithin defective graphene films. UV-Vis Spectroscopy measurements andband gap analysis were also conducted. To analyze the band gap energy(BGE) of the defective graphene, the samples supported on a givensubstrate were loaded into an Evolution 300 UVVis Spectrometer(ThermoFisher) equipped with a Praying Mantis Diffuse ReflectanceAccessory (Harrick Scientific) to measure the absolute diffusereflectance (R_(∞)), with a Spectralon® disk as a reference. The samplebeam is diffusely reflected off the sample, and the beam size is roughly1 mm². The absolute reflectance measured in the range from 190 nm to1100 nm is converted to Kubleka Munk units,F(R_(∞))=KMU=(1−R_(∞))²/2R_(∞), which is analogous to absorbance fordiffusely reflected samples. After measuring diffuse reflectance, theBGE of the samples was calculated through derivative peak fitting (DPR).Fityk software (Wojdyr, M. (2010) J. Appl. Cryst., J. Appl.Crystallogr., 43(5):1126-1128) was used to analyze the first derivativesof the absolute reflectance with respect to wavelength. Eachdifferential diffuse reflectance function was fit with Gaussian peaks,where each peak represents an independent BGE. The centers of each peakpresent in the differential plot correspond to potential band gapenergies of the sample.

To assess the applicability of high-defective graphene for use asfilters, highly defective graphene samples were fabricated intomembranes for ion selection property measurements. After hydrogenannealing, flame-synthesized graphene was transferred to a pinholesubstrate and suspended on the hole using the wet-chemistry method.Pinhole substrates used here were standard precision 5-μm roundapertures purchased from National Aperture. The permeability andconductance of defective graphene membranes were measured. For theconductance measurements, an electrochemical workstation using twoAg/AgCl electrodes (0.8 in diameter and 8 mm in length) and potassiumchloride aqueous solution (1 Mole/L KCl) was employed. Defectivegraphene membranes were attached to plastic holders with an innerdiameter of 5 mm and sealed with non-reactive epoxy for experimentation.The methodology used in the measurements was to apply voltages andrecord the corresponding current values using a 3 electrodeconfiguration (WE, CE and RE). The conductance of the tested membranewas calculated based on the I-V curve.

Results

In order to understand the mechanism of hydrogen annealing, graphenesamples on different substrate materials were first investigated. Theetching effect of hydrogen annealing makes it possible to create defectsin graphene film directly and tailor properties. Thus, hydrogenannealing conditions are varied to induce graphene defects. Thestructure and property of the as-grown defective graphene werecharacterized. Lastly, the potential applications of highly-defectivegraphene were explored.

Effect of Hydrogen Annealing on Graphene with Different Substrates

How hydrogen annealing affects the quality of graphene prepared ondifferent substrates was investigated. Flame synthesis of few-layergraphene (FLG) on nickel substrate was produced at a wide range oftemperature from 850° C. to 1000° C. A typical Raman spectrum of FLGgrown on 25 μm thick Ni foil is shown in FIG. 5. Unlike graphene grownon Cu, graphene quality on Ni is not affected much by the J_(CH4):J_(H2)ratio or temperature. Graphene growth on Ni is subject to a segregationmechanism because of the higher carbon solubility in Ni than in Cu.After post-growth hydrogen annealing, flame-synthesized FLG was fullyetched away on Ni. One explanation is similar to hydrogen annealing onCu such that hydrogen can etch away weak carbon bonds at edges andgrowth fronts. If that is the case, Ni should have a better catalyticperformance assisting hydrogen etching than does Cu because the generaldefective level of graphene is lower on Ni. Another possibility is thecarbon dissolution-precipitation mechanism where carbon atoms dissolveback into Ni, with precipitation of nickel carbides (NiC) occurringsimultaneously (Leong, et al. (2014) Nano Lett., 14(7):3840-3847). Thedissolution of carbon requires imperfection sites like defects anddangling bonds in graphene (Leong, et al. (2014) Nano Lett.,14(7):3840-3847). Here, the result can be a combination of both effects,explaining why a controllable improvement in the number of graphenelayers has not been observed on Ni substrates after hydrogen annealing.

Typical Raman spectrum of PLD-grown FLG exhibits a large I_(D)/I_(G)ratio and low I_(G)/I_(2D) ratio (FIG. 5). The presence of the 2D peakindicates the existence of graphene or graphitic structure. However,such spectrum also suggests the film consists of amorphous carbon andsmall domain graphene with defects because of the large I_(D)/I_(G)ratio and slight merging of D and G peaks. After hydrogen annealing,this highly defective FLG is etched away with no carbon signals detectedby Raman spectroscopy. It has been demonstrated that at hightemperature, hydrogen can react with carbon dangling bonds andeffectively etch away adlayers of graphene without noticeable damage tothe film. The removal of PLD-grown graphene on Si/SiO₂ reveals thathydrogen etching is not only effective on adlayers but also danglingbonds at edges and defects. If the defects of the graphene film reach toa certain level, then hydrogen annealing can affect the film by inducingeven more defects.

To verify that the wipeout phenomenon is not due to the use of anonmetal substrate, typical flame-synthesized BLG(I_(D)/I_(G)/I_(2D)˜0.4:1:1) films were transferred from Cu to Si/SiO₂for subsequent hydrogen annealing experiments. Interestingly, the BLGfilm was retained without clear improvement or reduction in qualityafter hydrogen annealing. No observational change in I_(G)/I_(2D) ratioindicates that the hydrogen annealing converting BLG to MLG on Cureported above is likely a metal-substrate-assisted process (Yao, etal., (2012) Carbon 50(14):5203-5209). The BLG film is not observed withany noticeable damage after annealing, meaning that suchflame-synthesized graphene does not contain considerable amounts ofdefect sites like PLD-grown graphene does to initiate the wipeoutprocess. Therefore, hydrogen annealing improves graphene quality on Cu,by etching away adlayers through a metal-assisted mechanism, but it canalso wipe out graphene films that contain vast amounts of defects.

Direct Synthesis of Highly Defective Graphene

The strong etching effect of hydrogen annealing on low-quality graphene(high I_(D)/I_(G) ratio) indicates that direct synthesis ofhighly-defective graphene with tunable defects may be synthesized bycontrolling the raw (starting) graphene quality and annealingconditions. To examine this potential, graphene with high I_(D)/I_(G)ratio (>0.4) was first intentionally synthesized with the m-IDFs burner.Here, methods for creating more defects in graphene, which holds greatpotential for many applications, such as gas separation and waterdesalination, via a controllable flame method are provided.

For a typical graphene sample (mono-, bi- or few-layer) synthesized inthe flame system, the I_(D)/I_(G) ratio is ˜0.4 (Memon, et al. (2011)Carbon 49(15):5064-5070). Hydrogen annealing does not observably damageflame-synthesized graphene with an I_(D)/I_(G) ratio of ˜0.4. The reasonis that the D peak in flame-synthesized graphene is mainly caused bysubmicron domain boundaries, not structural defects like vacancies. Theformation of the grain boundary is due to the termination of graphenegrowth when two neighboring grains meet. Compared with conventional CVDprocesses, flame synthesis utilizes much higher flow fluxes so that theflames can be stabilized, which results in more prolific nucleation ofcarbon atoms, with a higher density of nucleation sites, on thesubstrate surface. Graphene grown from domains with differentorientations normally do not merge to a single larger crystal, insteadgenerating a polycrystalline film. The I_(D)/I_(G) ratio offlame-synthesized graphene indicates the small grain size where thegrain boundaries create defective sites. The adlayers of graphene atgrowth fronts contains more edges and dangling bonds that can beeffectively etched away in hydrogen annealing, which, viewed in anotherway, enables the tailoring of the number of graphene layers.

Two parameters can significantly increase I_(D)/I_(G) ratio in graphene:substrate temperature and materials. By lowering substrate temperature,the number of disorder increases in graphene crystals, and amorphouscarbon growth occurs. Of course, amorphous carbon is not desired in thiswork since graphene structure needs to be maintained for manyapplications. By switching substrate from ultrapure Cu (99.9999%) to Cu(99.8%), the I_(D)/I_(G) ratio increase from ˜0.4 to ˜0.6, which meansthe impurity of Cu can increase the disorder in graphene grown fromscratch. For this reason, high-purity Cu is generally favored forgrowing high-quality graphene in conventional CVD (Murdock, et al.(2013) ACS Nano, 7(2):1351-1359). Defective graphene films (I_(D)/I_(G)ratio ˜0.6) were synthesized purposefully using Cu (99.8%) in themodified IDF burner, and then a post-growth hydrogen annealing wasperformed in-situ at 1000° C. in the same experiment using the samesetup. All graphene samples were grown at 1000° C. with a J_(CH4)/J_(H2)of 1:100 for 5 minutes. Typical Raman spectra of the defective grapheneafter treating with hydrogen annealing are shown in FIG. 6.Interestingly, after a 5-minute treatment, the I_(D)/I_(G) ratioincreases significantly from ˜0.6 to ˜1. The ratio is 1.2 after10-minute annealing. However, if the annealing time is extended to 12-15minutes, the I_(D)/I_(G) ratio slightly decreases in some cases. Ratiosover 1.2 were not observed for any cases studied. The rising ofI_(D)/I_(G) ratio confirms that more defects can be induced in graphenethrough hydrogen annealing, especially when the starting film is alreadyquite defective. One thing to notice is that the I_(G)/I_(2D) ratio isvery steady with respect to annealing time. The I_(G)/I_(2D) ratio is anindicator of the number of layers within graphene. However, in NPG, theRaman spectra of a suspended MLG can dramatically change after differentexposure time to oxygen plasma. The number of layers does not change inthe suspended MLG, but the chemical structure can change significantlyafter the plasma damage. The shape, wavenumber, and intensity of thegraphene 2D band are strongly impacted by not only the number ofgraphene layers but also the perfection of the graphitic chemicalstructure. The vacancy, divacancy, or even nanoscale pores can createmany kinds of structural defects and disorders in graphene. Ahigh-resolution TEM image shows the chemical structure offlame-synthesized defective graphene (FIG. 7). Many kinds of graphenestructural defects like vacancy, double vacancy, and Stone-Wales defectcan be clearly observed by comparing the enlarged TEM image of defectivegraphene and pristine flame-synthesized MLG at the same magnitude (FIG.7). The exact defect types exhibited in as-synthesized defectivegraphene is difficult to be identified clearly because of the limitationin the image resolution; nevertheless, an abundant variety of defectsexist. However, such subnanometer defects, in general, afford someunique properties that have great potential in a wide range ofapplications. The Raman spectra provide a qualitative assessment of thedensity and size of graphene defects. The steady I_(G)/I_(2D) ratio withgrowth time indicates that the carbon graphitic structure is retained ingeneral during hydrogen annealing. Thus, by utilizing the hydrogenannealing treatment, the annealing condition can be used to tune thedefects in flame-synthesized graphene films. Unlike the otherapproaches, such as ion irradiation and oxygen plasma bombardment, thisapproach offers a direct way to produce highly defective graphene withtunable defects.

Besides the direct observation of graphene defects, the properties ofas-grown defective graphene were investigated. UV-Visible Spectroscopywas used to determine the band gap structure of graphene in the presenceof defects. Band gaps in defective graphene samples are of interestbecause of the absence of a bandgap in perfect MLG, which cannot providethe on-off switching needed in transistor applications. Nonzero bandgapcan be created and tuned in BLG by applying an electric field oruniaxial strain (Castro, et al. (2007) Phys. Rev. Lett., 99(21):216802;Zhang, et al. (2009) Nature 459(7248):820-823; Ohta, et al. (2006)Science 313(5789):951-954; McCann, et al. (2006) Phys. Rev. Lett.,96(8): 086805; Gao, et al. (2011) J. Phys. Chem. C, 115(8):3236-3242).Doping is another promising way to open and engineer the bandgap ingraphene. Either substitutional doping of nitrogen and boron atoms ingraphene structure or adsorption of groups and molecules (e.g., H₂SO₄,N₂O₄, AuPt, Au₃Pt₃, etc.) can open the band gap of graphene (Akturk, etal., (2010) Appl. Phys. Lett. 96:081914; Gao, et al. (2011) J. Phys.Chem. C, 115(8):3236-3242). Highly-defective graphene can be anotheravenue to give band gap properties.

Firstly, UV-Visible spectroscopy was performed on as-synthesizeddefective graphene still attached to its Cu substrate. The band gapenergies were measured via the Diffuse Reflectance Derivative PeakFitting (DPR) method, where the center of each Gaussian peak fit to thefirst derivative, dR_(∞)/dλ, spectra corresponds to a band gap. Prior toband gap analysis, the background signal from the Cu substrate issubtracted from the absolute reflectance of the sample. The bandenergies of defective graphene in the low-energy range (800 nm to 1000nm) are shown in FIG. 8. In the mid-energy range, Cu exhibits a strongplasmonic resonance around 580 nm. Four individual band energies aredetermined after Gaussian peak fitting (Table 2). The overall band gapobserved is ˜1.4 eV, which is higher than that of NPG (<1 eV) made byother approaches. It is noted that with increasing defects, the area ofthe Gaussian peaks in the low-energy BGE region increases indicatingthat by creating more defects, band gaps can be opened in graphene. Theconcentration of these gaps seems to be correlated to the number ofdefects in the graphene.

TABLE 2 Band energies of defective graphene with different I_(D)/I_(G)ratio. Case Band A Band B Band C Band D 0 min (I_(D)/I_(G)~0.6) 1.46 eV1.39 eV 1.32 eV 1.28 eV 5 min (I_(D)/I_(G)~1.0) 1.42 eV 1.35 eV 1.30 eV1.27 eV 10 min (I_(D)/I_(G)~1.2)   1.4 eV 1.34 eV 1.30 eV 1.27 eV

In order to get better isolated diffuse reflectance spectra, as-growndefective graphene samples were transferred from Cu substrate to quartzor fused silica substrates, thus eliminating the strong signal at ˜580nm due to the plasmonic resonance of Cu. Analysis of the Kubelka Munk ofthese samples (FIG. 9) shows that the maximum absorption at ˜270 nm (4.6eV), which corresponds to the λ-λ* transition of the aromatic C═C bondin graphene (Su, et al. (2014) J. Phys. Chem. C, 118(23):12520-12525;Chang, et al. (2013) ACS Nano 7(2):1333-1341). The shift in the peakposition has been previously determined to correspond to the area of thearomatic system (Su, et al. (2014) J. Phys. Chem. C,118(23):12520-12525; Tolle, et al. (2012) Adv. Funct. Mater.,22(6):1136-1144). The result indicates that the redshift of the maximacould be used to monitor the increasing amount of defects in the sample.

An exception is observed on ultrasmooth Cu (99.8%). By performing thesame flame synthesis process, graphene grown on ultrasmooth Cu surfaceexhibits a remarkably large D peak with an I_(D)/I_(G) ratio ˜1.2, whichwas never observed on commercial Cu (99.8%) substrates (FIG. 10). Theelectropolished surface may contain residuals from electrolytes, whichinduces more defects in graphene growth. Another possibility is that thenucleation density of graphene is higher on a smoother surface. Afterhydrogen annealing for 10 minutes, the I_(D)/I_(G) ratio is reduced to˜1, and I_(G)/I_(2D) rate is slightly increased. At some locations,I_(G)/I_(2D) rate can be smaller than 0.7, indicating MLG. Therefore,hydrogen annealing cures some defects and reduces the number of layersin this highly defective graphene originally grown on ultrasmooth Cu.

The outcome of hydrogen annealing is a combination of several effects,including the interaction between hydrogen and carbon dangling bonds,the catalytic mechanism of the substrate, and the rearrangement ofcarbon atoms at high temperature. Based on these studies, such outcomeis highly depending on the original graphene chemical structure andsubstrate materials. Both MLG and highly defective graphene can beachieved by utilizing this treatment.

One of the main potential applications of highly-defective graphene isultrafiltration, such as ion selection, gas separation, waterdesalination, and even DNA sequencing. Porous graphene membranes exhibitorders of magnitude higher flow rates than commercial reverse osmosismembranes, as well as have excellent salt rejection properties(Rollings, et al. (2016) Nat. Commun., 7:ncomms11408). To study the ionrejection property of flame-synthesized defective graphene, aconductance measurement was performed. As-grown defective graphene filmsare transferred to round pinholes with 5 μm aperture. In themeasurements, conductance comes from three parts: a) the open area; b)the access resistance (effective for very thin pores/membranes likegraphene); and c) surface charge. The general formulation, taking intoaccount the contribution of the surface charge (Σ) is:

$G = {\kappa_{b}\left\lbrack {{\frac{4L}{\pi \; d^{2}} \times \frac{1}{1 + {4\frac{I_{du}}{d}}}} + \frac{2}{{\alpha \; d} + {\beta \; l_{du}}}} \right\rbrack}^{- 1}$

where κ_(b) is the bulk conductivity, L is the pore length, d is thepore diameter; l_(du) is the Dukhin length (which can be approximated by(|Σ|/e)/2cs, where e is the elementary charge and c_(s) is the saltconcentration); α is a geometrical prefactor that depends on the modelused; and β can be approximated to be 2 to obtain the best fittingagreement (Feng, et al. (2016) Nature 536:197-200). To eliminate thesurface charge effect, the molarity of the KCl solution is increased to1 Mole/L, for the graphene membranes tests. In this case, theformulation becomes:

$G = {\kappa_{b}\left( {\frac{4L}{\pi \; d^{2}} + \frac{1}{d}} \right)}^{- 1}$

for the estimation of the pore size based on the measured conductance ofthe membrane (Rollings, et al. (2016) Nat. Commun., 7:ncomms11408).Here, the first term is the bulk conductance, and the second term is theaccess resistance. From the I-V curve slope, the conductance can beobtained; and from the conductance, the effective open-area diameter, d,can be estimated for a known membrane thickness value. In theestimation, the thickness of 1 nm for graphene is used. For the controltemplate, 14.7 microns (thickness of the 5-μm pinhole) is used. Only theeffective diameter of the total open area can be estimated since thevalue of individual pore diameters and density of defects are unknown.The conductance results of the control sample (bare 5-μm pinhole) anddefective graphene membranes with different ID/G ratio are listed inTable 3. The estimated effective diameter of the control samplecalculated using the formula above is 4.93 μm, whose error is around 1%.Since the testing area is the same (i.e., 5 μm) for all the samples, theestimated open-area percentage is calculated by (d/c/c)², where d_(c) isthe effective diameter of the control sample. For the case ofI_(D)/I_(G) ˜0.6, the estimated open-area percentage is 0.000023%, whichimplies the membrane is nearly impermeable to the ions. As the I_(D/G)ratio increases, the estimated open-area percentage also increases. Forthe most defective case I_(D)/I_(G) ˜1.2, the estimated open-areapercentage is 0.8%. If the pore diameter is known and the pore sizes areuniform, the pore number can be estimated by this approach.

TABLE 3 Results of conductance measurements and estimated open-areadiameter and percentage respect to different I_(D)/I_(G) ratios.Open-area Case Conductance G (S) Effective diameter percentage Control1.21E−05 4.93 μm    100% I_(D)/I_(G)~1.2 4.53E−06 0.44 μm    0.8%I_(D)/I_(G)~1.0 1.57E−06 0.15 μm   0.09% I_(D)/I_(G)~0.6 1.62E−08 0.0024μm 2.3E−5%

The nitrogen permeability has been measured for all of the defectivegraphene membranes. However, nearly no nitrogen flow is observed in anysample. Many reports in the literature suggest that nitrogen is nearlyimpermeable in nanoporous graphene (NPG) because of the surfaceadsorption (Du, et al. (2011) J. Phys. Chem. C, 115(47):23261-23266;Kim, et al. (2013) Science 342(6154):91-95; Sun, et al., (2014) Langmuir30(2):675-682). The nitrogen impermeability of flame-synthesizeddefective graphene implies the size of defects is mainly sub-nanometer.Such property affords a wide range of applications in gas separation.Based on current results, flame synthesis can synthesize directlygraphene with tunable defects (number and size), affording a wide rangeof ion selection and gas separation membrane applications.

Post-growth hydrogen annealing was performed on graphene samples withdifferent original conditions and substrate materials, such as ultrapureCu, Cu, Ni, Si and ultrasmooth Cu. The etching effect of hydrogenannealing is predominantly a metal-assisted process because graphenetransferred to Si substrate stays unchanged before and after annealingtreatment. However, the etching effect can be very strong on graphenesample containing lots of defects, regardless the substrate material.Based on current results, the initial I_(D)/I_(G) ratio found in thiswork is ˜0.6. The I_(D)/I_(G) ratio increases dramatically afterhydrogen annealing when as-synthesized graphene on Cu exhibits aninitial ratio of at least 0.6. However, the I_(D)/I_(G) ratio does notchange obviously after annealing if the initial ratio is lower than 0.6.Inspired by such phenomenon, “low-quality” graphene was intentionallyproduced on low-purity Cu substrate, and then highly defective graphenefilms with tunable defective level are achieved by employing hydrogenannealing treatments. Current methods mainly use ion irradiation orplasma to damage CVD-synthesized graphene in order to create nanoscaleor subnanoscale defects. However, such methods requiring expensivesetups, and multiple steps may not be economically viable forlarge-scale production. The well-controlled flame method offers areadily available route to synthesize directly highly-defective graphenescalably, economically, and rapidly. By controlling the number ofdefects, a direct band gap can be opened up in flame-synthesizedgraphene. The band gap energy (BGE) was analyzed by UV-Vis spectroscopy.The chemical structure of as-synthesized highly defective graphene wasstudied using high-resolution TEM. Various types of defect includingpoint defect, divacancy, and Stone-Wales defect were observed. Suchsubnanometer defect sites contribute to the direct BGE, affording manyapplications. The filtration property of the highly-defective grapheneas a membrane was investigated, showing potential in ion selection andgas separation.

EXAMPLE 4

The exceptional physical and chemical properties of graphene afford awide range of applications from next-generation electronics to novelbiomedical devices. To enable the practical applications of graphene, anaffordable manufacturing method suitable for largescale production issought.

Flame synthesis has demonstrated its viability in growing few-layergraphene (FLG) using a dual flame setup or a multiple inverse-diffusionflame burner. However, the growth of mono- and bilayer graphene using anopen-atmosphere flame process remains challenging because of thebreakdown of self-limiting growth on copper as pressure is raised toatmospheric pressure. The synthesis configuration employed herein isbased on a modified multi-element inverse-diffusion flame (m-IDF) setup.The pyrolysis vapors and post-flame species are directed at a substrateto grow graphene. Each of the tiny individual flames operates in theinverse mode (“under-ventilated), where for each flame, the oxidizer(e.g., oxygen diluted with inert) is in the center and fuel (e.g.,hydrogen diluted with inert) surrounds it. Far downstream, multiplediffusion flames create a one-dimensional post-flame profile withradially uniform profiles of temperature and chemical species. Suchone-dimensional flame in net effect is suitable for the fundamentalstudy of graphene growth in flame. The precursor tubes inject precursorgases (e.g., methane, ethylene) downstream of the flame. This designensures that hydrocarbon precursor does not pass through or near themultiple flames, avoiding oxidation and dissociation of hydrocarbonspecies. Since carbon formation process and fuel oxidation process areeffectively separated, no soot is observed in the modified m-IDF setupfor all experimental conditions. Modified m-IDF has no scaling problemsince all flow velocity can be independent of the burner diameter. Suchtechnique affords large-area deposition of nanostructured carbon inopen-atmosphere by shielding the setup with an inert co-flow orencompassing tube preventing diffusion from the ambient.

Experiment

Copper (Cu) and nickel (Ni) foils from Alfa Aesar were used assubstrates for graphene growth. The metal foils were placed downstreamof the m-I_(D)Fs in the post-flame gases at a certain distance (8-12mm). Prior to graphene growth, the copper substrate was first reduced byrunning the m-I_(D)Fs with only hydrogen as fuel, such that thedownstream gases are rich in hydrogen at ˜1000° C., for 10 minutes toremove any oxide layer and to enlarge the grain size on the metal foils.No hydrocarbon species was introduced into the system during thisperiod. To initiate graphene growth, methane, as a precursor, is thenintroduced into the post-flame region through the precursor tubesdirected at the substrate. Bi- and few-layer graphene are grown oncopper and nickel substrates for different methane-to-hydrogen flow rateratios (JcH4:JH2), growth temperatures, and durations. An S-typethermocouple (125 μm Pt/Pt-10%Rh) coated with silica is utilized tomeasure the substrate temperature, which ranges from 800° C. to 1000° C.When the precursor injection period was done, the m-IDFs wereextinguished by shutting off the oxygen supply. Hydrogen and inert gasescontinued to flow until the substrate was cooled to room temperature.As-synthesized graphene can be transferred onto Si/SiO₂ substrates froma Cu substrate by following these steps: i) spin-coat one side of thegraphene/Cu/graphene sample with 300 nm-thick polymethyl-methacrylate(PMMA) film and heat in furnace at 150° C. for 5 minutes to cure thePMMA protection film; ii) immerse PMMA/graphene/Cu/graphene stack in 10%HNO₃ solution for 2 minutes to remove graphene on the unprotected sideand rinse in DI-water for multiple times; iii) float PMMA/graphene/Cufoil in 1 mole/L FeCl₃ solution to etch away all copper substrate; iv)rinse the PMMA/graphene film in DI-water for multiple times and thentransfer it onto Si/SiO₂; v) remove PMMA layer by rinsing in hot acetoneand dry at room temperature overnight. Micro-Raman spectroscopy(Renishaw 1000, 514 nm laser wavelength, 50×magnitude) is utilized tocharacterize the quality of graphene on Cu and Si/SiO₂ substrates, postexperiment.

Results

Flame synthesis of FLG has been demonstrated on different transitionmetals, but monolayer graphene (MLG) and bilayer graphene (BLG) have notbeen achieved using any open-atmosphere flame process. Although thethermodynamics of graphene grown on a Cu catalyst surface should be thesame for both atmospheric- and low-pressure chemical vapor deposition(CVD) processes, the appearance of bi- and few-layer graphene inatmospheric-pressure CVD process reveals that the kinetics (coolingrate, synthesis pressure, methane concentration) impact significantlythe thickness uniformity and quality of graphene growth (Bhaviripudi, etal. (2010) Nano Lett., 10(10):4128-4133). Therefore, herein, aparametric study was performed to optimize flame conditions for graphenegrowth. Raman spectroscopy enables ex-situ characterization of thequality of graphene, including the number of layers and defective level(Malard, et al. (2009) Phys. Rep., 473(5-6):51-87). For 514 nm laserexcitation, the typical Raman spectra of graphene have three prominentpeaks, the D peak at ˜1350 cm⁻¹, which corresponds to the disorderspresent in graphene layer, the G peak at ˜1580 cm⁻¹, and the 2D peak at˜2700 cm⁻¹. The intensity ratio of G-peak-to-2D-peak (I_(G)/I_(2D)) canbe used as a qualitative indicator to estimate the number of graphenelayers. For monolayer graphene, the I_(G)/I_(2D) ratio is usually aroundor above 2. The ratios are around 1 and smaller than 1 for bi-andfew-layer graphene respectively.

The formation of amorphous carbon films on Cu has been reported in thetemperature range between 500° C. and 750° C. using multipleinverse-diffusion flames (Memon, et al. (2013) Proc. Combustion Inst.,34(2):2163-2170). The 2D peak of graphene, which indicates the presenceof graphitic carbon structure, starts to appear on Cu at the temperatureof 700° C. (Memon, et al., Carbon 49(15):5064-5070). Typical Ramanspectra of graphitic carbon structures grown on Cu at low-temperaturerange from 700° C. to 850° C. The high I_(G)/I_(2D) ratio and themerging of D and G bands imply the growth of amorphous carbon andnanocrystalline graphite at low-temperature (Ferrari, et al. (2000)Phys. Rev. B, 61(20):14095-14107).

The effective synthesis of few-layer graphene (FLG) is observed atincreasing temperatures starting at 850° C. Raman spectra of graphenegrown at different temperatures for a fixed growth duration andJ_(C)H4:JH2 rate are shown in FIG. 11. By increasing synthesistemperatures from 850° C. to 1000° C., a significant decrease inI_(G)/I_(2D) ratio is observed from 2 to 1.2, which indicates the numberof graphene layers decreases as temperature increases. Graphene withI_(G)/I_(2D) ratio of 1.3 has been reported as a tri-layer film (Reina,et al. (2009) Nano Lett., 9(1):30-35), and 5 to 10 layers of graphenehas an I_(G)/I_(2D) ratio from 1.8 to 2.4 (Robertson et al. (2011) NanoLett., 11(3):1182-1189). A lower intensity of D peak indicates lessdisorder within the film. This result agrees with the consensus of CVDprocesses mostly using ˜1000° C. (Li, et al. (2009) Science324(5932):1312-1314; Li, et al. (2011) J. Am. Chem. Soc.,133(9):2816-2819; Xing, et al. (2013) Chem. Phys. Lett., 580:62-66),which is right below the melting point of copper (˜1085° C.), as anoptimum growth temperature.

In atmospheric-pressure CVD (APCVD), though not an open environment,monolayer graphene growth has been reported at very low J_(CH4):J_(H2)ratios (<1:1000) (Bhaviripudi, et al. (2010) Nano Lett.,10(10):4128-4133). However, the termination of FLG growth was observedwhen the methane-to-hydrogen flow rate ratio (J_(CH4):J_(H2)) is below1:40, using the multiple inverse-diffusion flames burner (Memon, et al.(2011) Carbon 49(15):5064-5070). In that flame synthesis experiment, CH₄was delivered through the fuel tubes, along with hydrogen, of the m-IDFsburner (and not by separate precursor tubes). Thus, the effect ofJ_(CH4):J_(H2) ratio is coupled with the flame condition because onlyexcess hydrocarbon species can serve as the active carbon source forgraphene growth. Therefore, the concentration of active carbon speciesin the post-flame region cannot afford graphene growth if theJ_(CH4):J_(H2) ratio is lower than a critical value. SinceJ_(CH4):J_(H2) ratio plays a major role in the kinetics of graphenegrowth, a parametric study of its effect is necessary to optimize theflame synthesis condition.

Taking advantage of the modified m-IDF setup (where extended precursortubes are utilized), the study of graphene growth was in a wide range ofJ_(CH4):J_(H2) ratios because the CH₄ flow rate is now independent ofthe flame condition. When J_(CH4):J_(H2) ratio is larger than 1:20,uniform FLG is grown on Cu, which agrees with results of FLG growth atJ_(CH4):J_(H2) ratios from 1:5 to 1:20 in flame synthesis (Memon, et al.(2011) Carbon 49(15):5064-5070). A wide range of J_(CH4):J_(H2) ratiosfrom 1:25 to 1:1000 was studied at a fixed substrate temperature of1000° C. and growth duration. Raman spectra of graphene grown on Cu withJ_(CH4):J_(H2) ratio varied from 1:25 to 1:100 is shown in FIG. 12. TheI_(G)/I_(2D) ratio decreases from 1.5 to 1.1 as the J_(CH4):J_(H2) ratiodrops. With decreasing methane flow rate, the density of graphenenucleation sites reduces because of the lessening of the degree ofsupersaturation of active carbon species on the copper surface topromote graphene nucleation. The reduction in nucleation density oftenleads to a higher quality growth of graphene with larger domain size andfewer imperfections. However, by further lowering methane concentrationto the condition of APCVD, it only leads to a longer time needed forgraphene to cover the substrate surface and MLG was not observed in anyof the cases. The case of graphene growth with J_(CH4):J_(H2) ratio at1:350 for 20 minutes is nearly identical to the case with J_(CH4):J_(H2)ratio at 1:100 for 10 minutes. The high flow flux and large numbers ofcombustion products (e.g., H₂O, OH, CO, CO₂) make a flame synthesisconfiguration different than CVD results. Such open-atmosphere flameprocess seems to be limited in reducing graphene nucleation sites.

Growth time is another critical parameter for the gas-phase synthesis ofgraphene at atmospheric pressure since the self-limiting mechanism isnot valid in the case. A larger number of graphene layers is expectedfor longer growth time because adlayers are formed simultaneouslybetween the first layers and the Cu surface. At a graphene nucleationsite, all adlayers share the same nucleation center and have the sameedge termination. In this experiment, the growth time varies from 30seconds to 30 minutes, while growth temperature is fixed at 1000° C.,and J_(CH4):J_(H2) ratio at 1:100. However, when growth time is reducedbelow 5 minutes, the graphene film cannot fully cover the Cu substrate(15 mm by 15 mm). Raman spectra in FIG. 13 shows the number of graphenelayers decreasing significantly based on I_(G)/I_(2D) ratio when thegrowth time decreases from 20 minutes to 5 minutes. For a 5-minutegrowth at optimal temperature (1000° C.) and J_(CH4):J_(H2) ratio(1:100), the I_(G)/I_(2D) ratio is less than 1, which suggests thegrowth of BLG, being achieved for the first time in an open-atmosphereopen-environment flame process. Nevertheless, no monolayer graphenegrowth is observed in the growth time between 30 second and 5 minutes.An explanation of this result is that a relatively high carbon fluxstill reaches the substrate, leading to small graphene domain size(Memon, et al. (2011) Carbon 49(15):5064-5070), where the time scale ofadlayer growth is on the order of graphene nucleation.

In the gas-phase synthesis of graphene, the growth mechanism stronglydepends on substrates materials. The solubility of carbon at hightemperature dominates the growth process. Graphene growth on transitionmetals has been demonstrated in various synthesis methods. However,metal-catalyst-free synthesis of graphene has very limited progressbecause of the weak adsorption of hydrocarbons on non-metals.Polycrystalline graphene on SiO₂ has been achieved using an oxygen-aidedCVD process performed at 1100° C. for 3 to 8 hours (Chen, et al. (2011)J. Am. Chem. Soc., 133(44):17548-17551). Such technique demonstrates thepossibility of directly growing graphene on Si/SiO₂ via CVD, but thegraphene film quality is not comparable to that of conventional CVD.Moreover, this oxygen-aided CVD method is not feasible for graphenemanufacturing because of the high energy consumption rate and lowproduction rate.

A number of substrate materials have been studied in this research.Raman spectra of graphene synthesized on nickel (99% Ni, 25 μm), copper(99.8% Cu, 25 μm), and ultrapure copper (99.9999% Cu, 0.1 mm and 0.25mm) substrates undergoing exactly the same experimental process aredisplayed in FIG. 14. All metal substrate materials were purchased fromAlfa Aesar. Both 0.1-mm and 0.25-mm thick ultrapure copper substratesgive identical results, indicating graphene growth on Cu is a surfaceprocess mostly independent of substrate thickness. BLG on ultrapure Cuhas noticeably better quality with higher I_(G)/I_(2D) ratio and lower Dpeak intensity. Because of the segregation mechanism, graphene grown onNi is a uniform few-layer film with fewer defects than that grown on Cu.

Methane (CH₄) is the most used hydrocarbon precursor in the gas-phasesynthesis of graphene because of its low pyrolysis rate (Qi, et al.(2013) J. Phys. Chem. C, 117(27):14348-14353). However, ethylene (C₂H₄)and acetylene (C₂H₂) have also been investigated for growing graphene inCVD (Qi, et al. (2013) J. Phys. Chem. C, 117(27):14348-14353; Luo, etal. (2011) J. Mater. Chem., 21(22):8038-8044; Ramon, et al. (2011) ACSNano, 5(9):7198-7204; Wang, et al. (2013) New J. Chem., 37(3):640-645).The effect of C₂H₄ and C₂H₂ on the flame synthesis of graphene wasinvestigated using the modified m-IDF burner.

Ultrapure Cu was used as a substrate for C₂H₄ and C₂H₂ experiments. Inboth experiments, the flow rate ratio between carbon precursor and H₂(C/H₂) was maintained at 1:50, which is equivalent to the caseJ_(CH4):J_(H2)=1:100 with respect to the total amount of carbon input.Raman spectra show BLG in C₂H₄ and C₂H₂ cases for a 5-minute growth timeat 1000° C. (FIG. 15). Such results are identical to a previous CH₄ caseunder the same growth condition. However, a lower growth rate has beenobserved in both cases. This result may be because the carbon flux ishalved to maintain the same C/H₂ input rate as the CH₄ case. The D peakto G peak ratio for both cases lies in between 0.3 and 0.4, which issimilar to that of the CH₄ case.

Flow field profile affects the diffusion of active carbon species fromthe post- flame gases to the substrate. Uniform graphene with grainsizes up to 10 μm has been grown on Cu foil tilted at an angle againstthe gaseous flow in a CVD process (Zhang, et al. (2012) J. Mater. Chem.,22(35):18283-18290). Here, substrates were placed in differentorientations in order to create different flow field profiles forgraphene growth. Raman spectra of graphene grown on Cu withperpendicular, tilted)(45°), and parallel orientations with respect tothe post-flame flow are shown in FIG. 16. The I_(G)/I_(2D) ratios are˜1, for all three orientations at optimal growth condition.Perpendicular orientation creates a stagnation flow profile, leading toa result with slightly lower I_(G)/I_(2D) ratio than do tilted andparallel orientations. The slight difference in graphene quality can bedue to the difference in mass transport in the different boundary layerflows.

Raman spectroscopy was employed on a bilayer graphene (BLG) before andafter a transfer to evaluate the transferability of flame-synthesizedgraphene. Raman spectra show equal I_(G)/I_(2D) ratio after transfer,which means the graphene film was successfully transferred from the Cuto Si/SiO₂ substrate using the wet-chemistry process. The D peakintensity is weaker on Si/SiO₂ substrate than on the original Cusubstrate, which implies a reduction in graphene defects induced bycopper surface imperfections such as grooves and grain boundaries. TheI_(D)/I_(G) ratio on Si/SiO₂ is ˜0.4, which agrees with FLG growth usingflame synthesis. The G and 2D bands get sharper and narrower as the fullwidth at half maximum decreases after transfer, which also implies thatsome copper surface effects are eliminated.

By employing the modified multi-element inverse-diffusion flames setup,the influence of various parameters, including substrate material,precursor, temperature, and growth time on graphene synthesis wasdemonstrated. Under optimized condition, the synthesis of bi-layergraphene was reported for the first time using an open-atmosphere flamesynthesis method. The transition growth from few-layer graphene tobilayer was observed by varying substrate temperature andmethane-to-hydrogen flow rate ratio. Graphene films are grown ondifferent substrates. Bilayer graphene films exhibit different levels ofdefect on Cu substrates with different purities. The higher the Cupurity; the fewer defects or disorders are induced by the substrate.Because of a different growth mechanism, Ni substrate gives thickergraphene growth (few-layer), but with fewer defects. The effects fromdifferent precursors and flow field were examined. Moreover, an in-situRaman spectroscopy measurement was performed on the experimental setupto understand the evolution of carbon species in the flame flow. Thedominant carbon species observed near the substrate are CH₄, C₃H₈, C₂,and CH₂, which can be the main carbon species transported to thesubstrate promoting graphene growth. Flame-synthesized graphene can betransferred onto arbitrary substrates for applications. Therefore, suchmethod can continuously produce bi- and few-layer graphene over largeareas, for example by rasterizing, in an open-atmosphere environment.Compared with conventional CVD, flame synthesis also has advantages inprocessing time and cost.

A number of publications and patent documents are cited throughout theforegoing specification in order to describe the state of the art towhich this invention pertains. The entire disclosure of each of thesecitations is incorporated by reference herein.

While certain of the preferred embodiments of the present invention havebeen described and specifically exemplified above, it is not intendedthat the invention be limited to such embodiments. Various modificationsmay be made thereto without departing from the scope and spirit of thepresent invention, as set forth in the following claims.

What is claimed is:
 1. A method for synthesizing a graphene structure byreacting an oxidizer and a fuel in a modified multiple,inverse-diffusion flame burner, wherein said modified multiple,inverse-diffusion flame burner comprises delivery tubes for the fuelextending beyond the main burner surface.
 2. The method of claim 1,wherein said graphene structure is selected from the group consisting offlakes, films, sheets, plates, and discs.
 3. The method of claim 1,where the modified, multiple, inverse-diffusion-flame burner comprisesan array of stabilized flames that form a uniform flat-flame front. 4.The method of claim 1, wherein the oxidizer is air, O₂, or an oxidizingagent.
 5. The method of claim 1, wherein said graphene structure ismonolayer graphene.
 6. The method of claim 1, wherein said graphenestructure is nano-defective graphene.
 7. The method of claim 1, whereinthe graphene structure is synthesized on a metal substrate.
 8. Themethod of claim 7, wherein the metal substrate is copper.
 9. The methodof claim 1, where the fuel contains at least one additive, therebyforming doped graphene structures.
 10. The method of claim 9, whereinthe additive is a nitrogen species or a boron species.
 11. The method ofclaim 1, further comprising assaying the graphene structure by Ramanspectra.
 12. The method of claim 1, further comprising transferring thegraphene structure to a substrate after synthesis.
 13. The method ofclaim 1, further comprising doping the graphene structure with ions. 14.The method of claim 13, wherein said graphene structure is doped withions in vacant atomic sites.
 15. The method of claim 13, wherein saidions are metal ions.
 16. The method of claim 5, wherein said methodcomprises: a) reducing a metal substrate in said modified, multipleinverse-diffusion flame burner wherein hydrogen (H₂) is the only fuel;b) adding a hydrocarbon precursor through the fuel lines extendingbeyond the main burner surface of the modified, multipleinverse-diffusion flame burner to synthesize the graphene on saidsubstrate; and c) annealing the flame-synthesized graphene of step b) byrunning the modified, multiple inverse-diffusion flame burner whereinhydrogen (H₂) is the only fuel.
 17. The method of claim 6, wherein saidmethod comprises: a) reducing a metal substrate in said modified,multiple inverse-diffusion flame burner wherein hydrogen (H₂) is theonly fuel; b) adding a hydrocarbon precursor through the fuel linesextending beyond the main burner surface of the modified, multipleinverse-diffusion flame burner to synthesize the graphene on saidsubstrate; and, optionally, c) annealing the flame-synthesized grapheneof step b) by running the modified, multiple inverse-diffusion flameburner wherein hydrogen (H₂) is the only fuel, wherein theflame-synthesized graphene of step b) has an I_(D)/I_(G) ratio of about0.6 or higher prior to step c).
 18. The method of claim 17, wherein saidmethod comprises annealing the flame-synthesized graphene of step b) byrunning the modified, multiple inverse-diffusion flame burner whereinhydrogen (H₂) is the only fuel.
 19. The method of claim 17, wherein saidmetal substrate is less than 99.9% pure.